Energy is released when two liquids of different salinities mix together. For example, the osmotic pressure difference between fresh water and sea water is approximately 29 atm at 20° C. For a flow rate of 1 m3/s this represents a theoretical power of almost 3 MW. This release of energy may therefore be used to generate power, for example at the mouth of a river as it enters the sea.
Most methods of extracting this energy rely on osmosis of water across semi-permeable membranes. One such method is pressure-retarded osmosis (PRO). In PRO a saline solution is contained within a pressure chamber and separated from fresh water by a semi-permeable membrane. The semi-permeable membrane is permeable to water but impermeable to the dissolved salt ions (Na+ and Cl−). The passage of water across the semi-permeable membrane from the fresh water side to the saline side causes the pressure in the chamber to increase. This pressure increase is then used to generate power, for example by releasing the pressure through a turbine to generate electricity.
Another method of generating power from salinity gradients is reverse electrodialysis. In reverse electrodialysis a saline solution and fresh water are passed through ion-exchange membranes. The chemical potential difference between the saline solution and fresh water generates a voltage across the membranes, thus providing power.
Both of these methods rely on the use of semi-permeable membranes and suffer from numerous drawbacks as a result. The disadvantages of using semi-permeable membranes include their high cost, their vulnerability to fouling, degradation, polarisation, the substantial head loss that occurs when a liquid passes through a membrane, and the requirement to filter and pre-treat the solutions.
An alternative method for generating power from salinity gradients is to use the free surface of the liquids themselves as the membrane. Since a saline solution has a lower vapour pressure than that of fresh water, water vapour will be transferred from fresh water to a saline solution in a sealed chamber. In Salinity Gradient Power: Utilizing Vapor Pressure Differences, Science, 206, 452-454 (1979) and Salinity-Gradient Vapor-Pressure Power Conversion, Energy, 7(3), 237-246 (1982) arrangements are described in which a turbine is interposed in the vapour flow between fresh water and a saline solution in an evacuated chamber, and it is suggested that the flow of vapour through the turbine could be used to generate power. In these arrangements the evaporation and condensation of the vapour causes a transfer of heat from the fresh water to the saline solution. It is therefore necessary to transfer heat back from the saline solution to the fresh water solution, otherwise the rate of vaporisation will reduce and eventually stop.
Although the vapour pressure methods outlined above overcome some of the disadvantages of using semi-permeable membranes, other drawbacks are associated with using such an approach. One disadvantage of these arrangements is that it is necessary for the atmosphere to be evacuated initially from the chamber to provide a vacuum in order that the flow of vapour may drive a turbine placed in the vapour flow. This requires an additional input of energy into the system and also requires that the liquid bodies are degassed to avoid outgassing into the evacuated chamber.
Another disadvantage of this prior art arrangement is that the pressure drop across the turbine is likely to be very small, approximately 0.4 mmHg, when vapour is transferred between fresh water and sea water at 20° C. This makes power extraction using a gas turbine impractical. The absolute pressure of the vapour is also low, approximately 18 mmHg at 20° C., which means that the force acting on the turbine is low, thereby hindering the amount of power that can be generated by the turbine.
It is an object of the present disclosure to provide improved apparatus and methods for generating power that utilise the flow of vapour between two or more liquid bodies having different vapour pressures.